Such a flexible pipe is for example fabricated in accordance with the documents relating to the standards API 17J (Specification for Unbonded Flexible Pipe) and API RP 17B (Recommended Practice for Flexible Pipe) established by the American Petroleum Institute.
The pipe is generally formed of a plurality of layers that are concentric and superposed. It is considered “unbonded” within the scope and purpose of the present invention since at least one of the layers of the pipe is capable of moving longitudinally in relation to the adjacent layers during the bending or flexion of the pipe. In particular, an unbonded pipe is a pipe that is free of any bonding materials that connect the layers forming the pipe.
The pipe is generally disposed across a body of water, between a bottom assembly, designed for collecting the fluid used at the bottom of the body of water and a floating surface assembly designed for collecting and distributing the fluid. The surface assembly may be a semi-submersible platform, an FPSO (Floating Production Storage and Offloading unit) or another floating unit.
In certain cases, for enabling the operational use of the fluids in deep waters, the flexible pipe has a length greater than 800 m. The ends of the pipe have end-pieces to be used for the connection to the bottom assembly and the surface assembly.
These pipe lines are subjected to very high axial tensile forces, in particular when the body of water in which the pipe is disposed is extremely deep.
In this case, the upper end-piece that connects the pipe to the surface assembly must absorb a very high degree of axial tension, which can reach several hundreds of tonnes. These forces are transmitted to the end-piece by means of the tensile armour layers extending along the pipe.
The axial tension not only has a high mean value, but also presents continuous and ongoing variations as a function of the vertical movements of the surface assembly of the pipe, under the effect of the agitation of the body of water brought about by the swell or by the waves.
The variations in axial tension may reach several tens of tonnes and be repeated on a continual basis during the service life of the pipe. Over a period of 20 years, the number of cycles thus may well go up to more than 20 million.
It is therefore necessary to ensure that the attachment between the layers of tensile armour and the body of the end-piece is particularly robust.
The patent document WO 2007/144553 describes an end-piece of the aforementioned type, wherein the reinforcing armour wires diverge in a helical manner away from the axis of the pipe, and then converge together in a helical manner towards the axis of the pipe in the chamber delimited by the end vault and the cover of the end-piece. This results in a “capstan” like effect, which locks the armour wires into position in the end-piece.
However, such an end-piece is not entirely satisfactory. In order to assemble the end-piece, it is necessary to move the armour wires away from the axis of the pipe, and then to bring them close together towards the axis of the pipe, which results in a hardening of each armour wire at the point of separation. This hardening is quite likely to lead to a localised weakening of the bond, which may prove to be detrimental over the course of time, in particular when the end-piece is subjected to repetitive variations in axial tension.
In order to alleviate this drawback, the document WO 2007/144553 proposes to arrange a collar around the reinforcing armouring without unduly tightening it so as to limit the radial deformation of each armour wire during the assembly.
Such a solution is however not sufficient in some cases, in particular when the pipe gets subjected to a number of cycles of axial tension variations.